Sealing ring assembly and mounting method

ABSTRACT

A sealing ring assembly and an improved method for mounting a sealing ring into an electrochemical cell used for Electrochemical Capacitance Voltage (ECV) profiling measurements. The ring is located in a holder having at least one secondary bore providing fluid communication between a forward face of the holder and the central bore of the ring, directed parallel to but tangentially offset relative to the inner wall of the central bore so as to impart a degree of rotational flow to electrolyte entering the sealing ring through the or each secondary bore which effectively removes gas bubbles and refreshes the electrolyte. The holder facilitates ring removal with a much reduced risk of damage to the delicate sealing surface.

CROSS REFERENCE TO RELATED APPLICATIONS

This Application is a continuation of International Application No.PCT/GB2003/002819 filed on 30 Jun. 2003. Application PCT/GB2003/002819claims priority from Application 2016605.6 filed on 17 Jul. 2002 in theUnited Kingdom. The entirety of each of these applications isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to a sealing ring assembly and an improved methodfor mounting a sealing ring into an electrochemical cell so as toimprove the repeatability of electrochemical capacitance voltage (ECV)measurements.

Semiconductor devices are made by sandwiching layers of material ofdifferent electrical and/or optical properties together. The layers areformed by epitaxial growth on or ion implantation or diffusion into asubstrate wafer. Correct device operation necessitates close control oflayer properties including carrier concentration and thickness. U.S.Pat. Nos. 4,028,207 & 4,168,212 describe the ECV profiling method, whichis used for determining the carrier concentration as a function of depthinto the layer and is therefore ideally suited to measuring theseparameters.

ECV profiling makes use of the diode structure formed when a conductingliquid (electrolyte) is placed in contact with a semiconductor. Thecapacitance of the junction, in the reverse bias region, is determinedby the magnitude of the applied bias and the carrier concentration vs.depth profile. By measuring this capacitance as a function of bias thecarrier concentration depth profile can be determined. In this mode ofoperation the ECV profiler is similar to tools, which use metallisationor mercury in place of an electrolyte to form a diode structure.However, such tools are usually restricted to shallow depth profiles dueto reverse bias breakdown of the semiconductor-metal junction. By usingan electrolyte this limitation is overcome. The electrolyte is used toelectrochemically etch into the sample, increasing the depth profiledwithout increasing the measurement bias. This makes ECV profiling a verypowerful method for characterizing multi-layer structures.

ECV profiling is carried out using electrochemical cells of designswhich will be familiar to the skilled person. The cell provides aconducting electrolyte reservoir for the profiling process and includessuitable reference and counter electrodes. Electrolyte is caused tocontact with a semiconductor sample under test, which becomes theworking electrode of the cell. Usually a chemically resistant plasticring, referred to as a sealing ring, defines the area of contact betweenthe electrolyte and the semiconductor sample. Any damage to the sealingedge of the ring has a big effect on the measurements. In extreme casesthe seal will leak and no measurement is possible. Usually the sealingring is pressed into a counter bore in the electrochemical cell body.This action involves applying pressure close to the seal and there isalways the possibility of contacting the seal and damaging it.

Accurate measurements depend on knowing the precise area of contactbetween the test sample and the electrolyte and ensuring that it remainsstable throughout the measurement process. Some seepage under the lip ofthe ring is inevitable and this will give rise to errors in theanalysis, but the design of the ring is such that this is minimized andstable for any given electrolyte.

The sealing ring is made by injection moulding and its small size makesit difficult to handle and avoid accidental damage to the sealing edge.Ideally the sealing edge should make a knife edge contact with thesample although some compliance is essential to allow for slightmisalignment or non planer samples. However to avoid excessivedeformation, the edge has to be sufficiently rigid and this is achievedby employing a cone like structure with the material progressivelygetting thinner towards the sealing edge. The bore of the cone is alsotapered to ensure the surface of the sample can be uniformly illuminatedthrough the electrolyte.

Gas bubbles can affect the measurement during profiling. Gas bubblesarise due to trapped air when the electrochemical cell is initiallyfilled, or during electrolyte circulation or are a result of theelectrochemical reactions used to etch the sample. Gas bubbles lead tonon-planer etching which changes the measurement area and can seriouslyaffect measurement quality when multi-layer structures or structureswith changing carrier concentration are being profiled. The bubbles canbe removed by circulating the electrolyte in the cell, but the shape ofthe sealing ring makes this difficult, giving rise to dead spots wherethere is little or no flow allowing bubbles to accumulate.

In many prior art systems the semiconductor sample is mountedvertically. This helps prevent reaction products dropping back onto thesurface of the sample and makes it easier to drain the electrochemicalcell at the end of the measurement. However in this orientation gasbubbles are more prone to collect in the ring and affect the ECVprofiling measurement.

Electrochemically etching occurs when the sample is the anode (biasedpositively with respect to a counter electrode). For p-type materialthis is the forward bias condition and electrochemical etching readilytakes place. For n-type material this is the reverse bias condition andelectrochemical etching only occurs when the sample is illuminated withlight of energy above the band-gap of the material. The requirement toilluminate the sample surface for n-type materials means that any formof pumping system used to circulate the electrolyte in the ring, mustnot obstruct the illumination system.

Several schemes for removing bubbles and reaction products have beendescribed. German Pat. Nos. DE3103611 discloses a device, which usesultra-sound to perform this task. A modified sealing ring design isshown in “I. Mayes—Electrochemical C-V Profiling of Silicon”, ECSSymposium on Diagnostic Techniques for Semiconductor Materials andDevices, 1992, Vol 92-2, p249-260”. The sealing ring uses threetangential jets spaced 120 degrees apart to generate swirl in theelectrolyte and sweep bubbles and other reaction products away from thesurface of the sample, which in this case is mounted horizontally.

However this design is not suitable for vertically mounted samples. Gasbubbles tend to collect behind the jets and are not removed unless theelectrolyte flow rate is very high. In practice high flow rates aredifficult to achieve. Peristaltic and diaphragm pumps tend to produceundesirable pressure fluctuations. A magnetically coupled centrifugalpump meets the requirement for the pump to be chemically inert,non-shedding and be capable of being easily drained. Such pumps areprone to cavitation at high speeds and their use in this application istherefore restricted to reduced flow rates.

BRIEF SUMMARY OF THE INVENTION

Embodiments of this invention provide a sealing ring assembly and animproved method for mounting a sealing ring into an electrochemical cellwhich mitigates some or all of the above disadvantages, and inparticular which is more effective in removing bubbles and reactionproducts and/or is less vulnerable to damage to the sealing edge.

Exemplary embodiments of the invention provide a sealing ring andsealing ring assembly incorporating means of circulating the electrolyteso that any trapped air or gas generated during etching is swept out ofthe system and does not interfere with the ECV profiling measurement.

Some aspects of the invention provide a sealing ring and sealing ringassembly which is removable from the electrochemical cell, forinspection and cleaning, with reduced risk of damage to its delicatesealing edge, thus improving the reproducibility of measurements made byECV profiling.

Thus, in accordance with the invention in a first aspect, a sealing ringassembly for fitment to an electrochemical cell used for ECV profilingof a semiconductor sample to define a contact area between electrolytewithin the cell and the semiconductor sample comprises a sealing ringhaving a generally axial central bore, and a sealing ring holder adaptedto engage upon an electrochemical cell and so retain the sealing ring inposition, wherein the holder is provided with at least one secondarybore providing fluid communication between a forward face of the holderand the central bore, the or each secondary bore being directed parallelto but tangentially offset relative to the inner wall of the centralbore so as to impart a degree of rotational flow to electrolyte enteringthe sealing ring through the or each secondary bore.

In use, the sealing ring is fitted to an electrochemical cell. Thesecondary bore(s) serve as nozzle(s) through which the electrochemicalcell is filled with electrolyte. The nozzles are directed parallel tobut offset tangentially relative to an axial direction of the ring, soas to impart a degree of rotational flow to the electrolyte. As a resultair trapped in the sealing ring's bore is flushed out. The primary boreserves in conjunction with a suitably configured cell, and in particulara vertical cell, to provide a means of allowing gas bubbles to rise andreadily escape from the surface of the semiconductor sample.

Select embodiments of the invention have two advantages over existingmethods. They make the ring easier to change with a much reduced risk ofdamage to the delicate sealing surface. These embodiments alsoincorporate an electrolyte circulation mechanism which effectivelyremoves gas bubbles and refreshes the electrolyte.

It is a significant advantage of embodiments of the present inventionthat sealing rings may be used of generally conventional design. Theinvention lies in the use of the holder which allows for theintroduction of one or more secondary bores designed in use to serve asdirectional nozzles to produce the desired rotational flow. Since thedesired rotational flow is produced as an inherent feature of thesealing ring assembly, additional pumping mechanisms designed to producethis need not be incorporated into the cell.

Moreover, prior art sealing rings have generally been adapted for pressfit engagement into a receiving aperture. This does not assist readyremoval, as the delicate sealing edges can become damaged. The presentinvention may also provide for easy removal of the sealing ring from theelectrochemical cell for replacement, inspection and cleaning, and insome instances also to allow interchange with rings having differentsecondary bore designs to generate alternative flow patterns. Thesealing ring holder can reduce risk of damage to the delicate sealingedge.

Sealing ring assemblies in accordance with embodiments of the inventioncan be used in conjunction with many conventional cell designs withoutneeding major design modifications, and in particular are suited tovertical cell designs.

The sealing ring holder has cell engagement means to engage with anelectrochemical cell suited for ECV profiling, and means to receive asealing ring in so as to position the same in correct location relativeto the cell when the cell engagement means are engaged thereon.

At least one and preferably both of the cell engagement means andreceive sealing ring receiving means are adapted for ready removal, toallow the ring to be removed from the cell. The sealing ring receivingmeans are adapted to allow ready removal of the ring without damage tothe sealing edge, for example providing for slideable engagement of thesealing ring within a suitable housing. Preferably the sealing ring isheld in the holder by overfolds at the edges of the recess in which thering sits.

The cell engagement means may facilitate engagement of the assembly ofsealing ring and holder via sliding, twist-slide, screw or interferencefit or combination thereof.

In accordance a preferred embodiment the holder has a part which can begripped and is designed such that the holder can be placed in andremoved from the electrochemical cell without damaging the seal.

Sealing rings of generally conventional shapes may be readily adapted bymeans of provision of a holder incorporating secondary nozzle(s) in theholder to constitute sealing ring assemblies in accordance with theinvention. Thus, suitable structures will readily suggest themselves. Inparticular, sealing rings may have a cylindrical, conical orfrustoconical shape and be provided with a central axial bore, which maybe tapered or of constant diameter.

Most preferably, the body of the sealing ring is frustoconical, with theinner bore of the ring is tapered towards the sealing edge with thewidest part being away from the sealing edge on the back face of thering. In this arrangement, the one or more nozzles are directed parallelto the inner cone of the ring and tangential to its perimeter, such thatthe liquid in the cone is forced to rotate about the cone's major axis.The resulting flow removes reaction products, replenishing theelectrolyte and dislodging gas bubbles from the surface of the sampleand the inner wall of the ring.

The holder is provided with a sealing ring receiving cavity suitablysized and shaped to receive the sealing ring in snug engagement, andspecifically with a generally fluid tight contact between the commonfaces lying adjacent to maintain a leak proof fluid path throughsecondary and primary bores. In particular, the holder may have asuitably sized and shaped sealing ring receiving cavity and/or befabricated from suitable material and/or include suitable urging meansto urge the sealing ring into such generally fluid tight contact.

The secondary bore(s) making up the nozzles which impart fluid rotationmay be tapered or of constant diameter. Preferably, the secondarybore(s) are of constant diameter.

Preferably, a plurality of secondary bores are provided, disposed to liein suitable array around the sealing ring assembly. For example aplurality of bores may be disposed so that the nozzles they present atthe surface of the holder form at least one circular or arcuate array,the nozzles preferably being evenly spaced therearound. Additionalnozzles may be used to increase flow rate or alter the flow pattern.

In a preferred embodiment the nozzles are positioned in an arcuate arrayextending across no more than half a circle. This can make it easier toavoid interference between the flow they generate and the gas bubbleclearance route. In particular, for use in a cell adapted for verticaloperation, the nozzles are positioned in an arcuate array to extend inuse across the lower half of the sealing ring to avoid them interferingwith gas bubble clearance or acting as bubble traps.

The ring and holder may be fabricated from suitable known materials,such as plastics, natural or synthetic rubbers and the like. Thematerial requires chemical resistance to resist the electrolyte, eitherinherent to the material or in the form of a suitable coating.

The ring is fixed into the holder, which both locates the ring and alsocontains an aperture or apertures comprising the nozzles for directingthe electrolyte flow and facilitating fluid communication therethrough.In a preferred embodiment the ring holder also has a cavity in its rearface to allow gas bubbles to rise and be removed from the inner bore ofthe ring.

In a simple embodiment of the invention it is assumed that the edge ofthe sealing ring (sealing surface) and the sample are sufficientlyparallel so that a leak will not occur. Usually this is so, but it isprudent to allow for some self alignment of this assembly to offsetalignment errors in the machine and possible problems with the sample.

The sealing pressure is very small, so that self alignment of the wholeelectrochemical cell or wafer chuck, holding the sample, is notpractical. Very weak springs would be involved and are not appropriatefor these larger and heavier assemblies. It makes more sense toincorporate an alignment mechanism into the sealing ring assemblydescribed above.

Accordingly, in a preferred embodiment the sealing ring is structurallyadapted to facilitate a degree of self alignment in use. In particularthis is achieved in that the sealing ring is a composite structuregiving a degree of compliance at the sealing surfaces, for example asandwich structure comprising a relatively rigid primary sealing ring(generally as above described) additionally sandwiched on either side byrelatively compliant chemically resistant secondary rings. The secondaryrings may for example be fabricated from softer silicone or the like.

In use the secondary rings are placed above and below the harder (muchless compliant) sealing ring and this sandwich is then retained in theoriginal plastic holder. When placed into the holder the secondary ringsare slightly compressed by the action of folding over the edges of therecess in the holder. This prevents leakage. These soft rings are thusable to give a degree of effective self-aligning to the overallcomposite ring sandwich by accommodating small amounts of non parallismbetween the sealing ring and the sample. The sealing ring is effectivelyfloating in the holder, not in hard contact with any part. If anon-parallel sample initially contacts one side of the sealing ring theforce on this side will cause the ring to move in the holder until it ismore parallel.

In a further aspect of the invention an electrochemical cell used forECV profiling of a semiconductor sample comprises a sealing ring or asealing ring assembly as above described incorporated thereupon.

In a further aspect of the invention, a method for improving thecirculation of electrolyte in an electrochemical cell used for ECVprofiling of a semiconductor sample comprising:

-   locating a sealing ring assembly upon the cell to define a contact    area between electrolyte within the cell and the semiconductor    sample, wherein the assembly comprises a sealing ring provided with    a generally axial central bore and a sealing ring holder adapted to    engage upon an electrochemical cell and so retain the sealing ring    in position, the holder being provided with at least one secondary    bore forming a nozzle in the holder providing fluid communication    between a forward face of the holder and the central bore of the    ring, and being directed parallel to but tangentially offset    relative to the inner wall of the central bore;-   associating a semiconductor sample with the cell in suitable    position as to lie in electrical contact with the electrolyte in    use;-   using the nozzle(s) to fill the electrochemical cell with    electrolyte, so that electrolyte is forced to rotate about a central    axis of the ring and air trapped in the sealing bore is flushed out.

In a preferred embodiment, the sealing ring is first associated with thesealing ring holder, for example being inserted in a retaining cavitytherein, and the holder then mounted upon the cell to retain the ring inappropriate orientation.

In particular, the cell is adapted for vertical operation, and thesealing ring holder comprises a plurality of nozzles positioned in anarcuate array extending across no more than half a circle across thelower half of the sealing ring.

Preferably there is also provided, integrally or additionally to thering and/or holder, a means of allowing gas bubbles to rise and readilyescape from the surface of the semiconductor sample and the inner boreof the sealing ring. In particular, an obround slot with its major axisvertical is used to increase the electrolyte flow rate towards the topof the ring to allow gas bubbles to escape more readily.

In accordance with the method a suitable holder is used in conjunctionwith a plastic sealing ring which may be of conventional design. Theinner bore of the ring is preferably tapered towards the sealing edgewith the widest part being away from the sealing edge on the back faceof the ring.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagrammatic illustration of a ring holder in which asealing ring is mounted.

FIG. 2 shows a diagrammatic illustration of an electrochemical cell bodyhaving a slot in which the ring holder is located.

FIG. 3 shows a diagrammatic illustration of a sealing ring held in arecess in the ring holder by crimping.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the figures and firstly to FIG. 1 there is shown adiagrammatic illustration of an apparatus in accordance with theinvention.

The apparatus essentially comprises a ring holder [1] in which ismounted a sealing ring [2]. The front of the holder has a recess [3] tolocate the ring and hold it in place. The back of the holder containsthe electrolyte delivery system [6][7] and an exit port for theelectrolyte and bubbles [5]. A finger hole [4] to one side of the holderprovides a simple means of removal from the electrochemical cell withoutdamage to the sealing ring.

In the embodiment shown in FIG. 1, the ring holder is made from achemically inert plastic and the ring is crimped into the holder byfolding over the sides of the recess. The holder is designed to locatein a slot [8] in the electrochemical cell body [9], see FIG. 2. An ‘O’ring [10] in the cell body ensures a liquid tight seal with the rearface of the sealing ring holder [11]. The outer edge of the rear face ofthe holder is radiused to avoid damage to avoid damage to the ‘O’ ringwhen the holder is slid into or removed from the electrochemical cellbody. Electrolyte enters the ring holder through an aperture [12] in theelectrochemical cell body and leaves the ring holder, returning to thecell by a second aperture [13].

In the embodiment shown in FIG. 1 the sealing ring is represented forsimplicityas a unitary construction. In practice a sandwich constructionwith a realtively rigid cebtral ring between more compliant outer ringswill be preferred. As this is received in the holder the outer rings areslightly compressed by the action of folding over the edges of therecess in the holder. This prevents leakage and ensures in particularthat if a non-parallel sample initially contacts one side of the sealingring the force on this side will cause the composite ring to move withinthe holder until it is more parallel.

In the embodiment shown in FIG. 1, three nozzles [6] are positionedtowards the bottom of the sealing ring [2] in a recess [7] thatdistributes the electrolyte to the nozzles. They are angled so that theyare parallel and tangential to the inner wall of the ring so as toimpart a slightly rotational flow to the electrolyte entering thesealing ring through them. Because the holder by necessity is made ofthin material the diameter of the nozzles is small to preserve thecorrect angle. However it is within the scope of the invention to usemore or fewer nozzles to control either the flow rate, flow pattern orboth.

In a preferred embodiment of the invention the exit port [5] isobrounded with its major axis vertical to allow gas bubbles to rise andreadily escape. The obround is positioned so as not to obstruct uniformillumination of the sample. The obround is not centred on the seal, butis offset so that there is space for gas bubbles to rise and leave thering, without being trapped against the holder.

In a further embodiment of the invention the sealing ring [2] is held ina recess [3] in the holder [1] by folding over the sides of the recessso that the ring is crimped. This is achieved by using a tool whichheats the plastic, forms it into position and cools the plastic so thatit is set in the new position, see FIG. 3.

Other preferred features of the method will be understood by analogywith the foregoing.

1. A sealing ring assembly for fitment to an electrochemical cell usedfor ECV profiling of a semiconductor sample to define a contact areabetween electrolyte within the cell and the semiconductor samplecomprises a sealing ring having a generally axial central bore, and asealing ring holder adapted to engage upon an electrochemical cell andso retain the sealing ring in position, wherein the holder is providedwith at least one secondary bore providing fluid communication between aforward face of the holder and the central bore, the or each secondarybore being directed parallel to but tangentially offset relative to theinner wall of the central bore so as to impart a degree of rotationalflow to electrolyte entering the sealing ring through the or eachsecondary bore.
 2. A sealing ring assembly in accordance with claim 1wherein a plurality of secondary bores are provided, disposed insuitable array around the sealing ring.
 3. A sealing ring assembly inaccordance with claim 2 wherein the secondary bores are disposed so thatthe nozzles are positioned in at least one circular or arcuate array. 4.A sealing ring assembly in accordance with claim 3 wherein the nozzlesare positioned in an arcuate array extending across no more than half acircle adapted in use to extend across the lower half of the sealingring.
 5. A sealing ring assembly in accordance with claim 1 wherein thesealing ring holder has cell engagement means to engage with anelectrochemical cell, and means to receive the sealing ring so as toposition the same in correct location relative to the cell when the cellengagement means are engaged thereon.
 6. A sealing ring assembly inaccordance with claim 5 wherein the sealing ring is held in the holderby overfolds at the edges of the recess in which the ring sits.
 7. Asealing ring assembly in accordance with claim 1 wherein the holder hasa portion which can be gripped and is designed such that the holder canbe placed in and removed from the electrochemical cell without damagingthe seal.
 8. A sealing ring assembly in accordance with claim 1 whereinthe holder comprises a cavity in its rear face to allow gas bubbles torise and be removed from the inner bore of the ring.
 9. A sealing ringassembly in accordance with claim 1 wherein the ring has a cylindrical,conical or frustoconical shape and is provided with a central axialprimary bore.
 10. A sealing ring assembly in accordance with claim 9wherein the ring has a frustoconical body with the inner bore of thering is tapered towards the sealing edge with the widest part being awayfrom the sealing edge on the back face of the ring.
 11. A sealing ringassembly in accordance with claim 1 wherein the ring has a frustoconicalbody with the inner bore of the ring is tapered towards the sealing edgewith the widest part being away from the sealing edge on the back faceof the ring.
 12. A sealing ring assembly in accordance with claim 1wherein the ring is structurally adapted to facilitate a degree of selfalignment in use.
 13. A sealing ring assembly in accordance with claim12 wherein the sealing ring is a composite sandwich structure comprisinga relatively rigid primary sealing ring additionally sandwiched oneither side by relatively compliant chemically resistant secondaryrings.
 14. An electrochemical cell used for ECV profiling of asemiconductor sample comprising a sealing ring assembly in accordancewith claim 1 incorporated thereupon.